A new method is described for regioselective partial deprotection of carbohydrates protected as benzylidene acetals. This deprotection was accomplished for each of the six methyl pyranosides (4, 5, and 18-21) studied by irradiation of the protected sugar and .¡V-bromosuccinimide (NBS) in the presence of water. Under these conditions the benzylidene (1,3-dioxolane) ring in each compound opened to give a methyl pyranoside with an axial benzoyloxy group and an equatorial hydroxy group. For example, irradiation of methyl 3,4-O-benzylidene {R or S)-6-deoxy-2-0-(2,2-dimethylpropanoyl)-a-L-galactopyranoside (18 or 19) with NBS, barium carbonate, and water resulted in the formation of methyl 4-0-benzoyl-6-deoxy-2-0-(2,2-dimethylpropanoyl)-a-L-galactopyranoside ( 22) in 72% yield. In a similar manner compounds 4 and 5 gave 10 and compounds 20 and 21 produced 23. The advantages of this deprotection process are described.

Aus den Benzyliden‐acetalen (I) erhält man mit NBS über (II) fast ausschließlich das Spaltprodukt (III).